Adhesion of two surfaces can be enhanced by primers, coupling agents and other surface treatments. Because of the unique properties associated with certain non-wood materials such as metals, composites, ceramics and plastics, adhesion enhancement is often essential for developing highly durable bonds between the two surfaces. In those industries such surface treatments are commonplace.
In the lumber industry, wood products are frequently adhesively bonded. However, the performance of adhesives in wood products is decreased if the wood product has first been treated or if the wood will be used in wet conditions. For example, epoxy adhesives provide a bond that is as strong as the wood itself as long as the wood remains dry in service. The main disadvantage of using epoxy adhesives with wood products is that the adhesive bond delaminates in wet-use conditions. Therefore, epoxies have not heretofore been used to laminate or repair timbers if the bonds are to be subjected to shear and tension loading. Clark and Nearn (1957) and Williamson and Nearn (1958) investigated formulations, assembly factors, temperatures of cure, wood species and their densities as they affected the strength and durability of epoxy adhesives made from epichlorohydin and bisphenol A resin. In the early 1960's, improved epoxy formulations were developed that were capable of withstanding more severe exposure conditions Olson and Blomquist (1962). In the late 1960's, more apparent progress was made developing durable epoxy bonds using polyethylenimine primer by the Weyerhaeuser Company and the Dow Chemical Company Caster (1980).
Water-based wood adhesives such as phenolics, resorcinolics, isocyanates, ureas and melamines perform well only on wood that has not been chemically or physically altered. Wood treated with an inorganic preservative, such as chromium-copper-arsenate (CCA), is difficult to bond effectively for wet-use applications. The surface of CCA-treated wood is covered with insoluble and chemically fixed deposits of chromium, copper and arsenic. The deposits physically block polar adhesives from chemically bonding to wood which is normally polar. Vick and Kuster (1992) suggested that adhesively bonded products made from lumber, veneer, flakes and fibers can play a prominent role in the market place if difficulties in bonding preservative-treated wood can be overcome.
Phenolic-based adhesives have heretofore been used in the rubber industry. For example, U.S. Pat. Nos. 4,238,595 and 4,246,144 to Girgis describe phenolic aldehyde resins used for adhesive coatings to bind substrates such as glass fibers to rubber. Solomon (1990) suggested incorporating a resorcinol-formaldehyde thermosetting resin into a rubber-latex tire-cord adhesive. However, to date the use of a resorcinol-formaldehyde coupling agent to assist in providing greater adhesion by epoxy, phenol-resorcinol, or isocyanate-based adhesives between wood surfaces has not been shown.